This invention relates to a process for reducing the concentration of nitrogen oxides contained in a gaseous mixture. In particular, this invention relates to a process wherein the concentration of nitrogen oxides is reduced by catalytic reduction.
Nitrogen oxides are, of course, generally present in significant quantities in gaseous mixtures such as flue gases. Different methods have been used in the treatment of these gas mixtures. One type of treatment involves the catalytic reduction of the nitrogen oxides. As processes for catalytic reduction, two methods are known in the art: (1) a non-selective reduction method in which carbon monoxide, hydrogen or a lower hydrocarbon is used as the reducing agent, and (2) a selective reduction method in which ammonia is used as the reducing egent. The latter catalytic reduction method (using ammonia) is advantageous in that the amount of the reducing agent used can be reduced and nitrogen oxides can be removed at a high ratio. Accordingly, various modifications and improvements have been proposed on this method.
These reduction methods using ammonia are roughly classified into two groups, namely one in which a noble metal component such as platinum, palladium, rhodium, or iridium is used as the catalyst and another in which a base metal, particularly a non-noble transition metal component such as copper, iron, vanadium, chromium or molybdenum is used as the catalyst.
Noble metal catalysts are defective in that they are drastically poisoned by sulfur oxides generally contained in exhaust gases, and base metal catalysts are defective in that severe reaction conditions such as elevated reaction temperatures and reduced space velocities should be adopted in order to improve their activities. In view of the fact that exhaust gases discharged in large quantities from boilers which have now a very large scale should be treated for removal of nitrogen oxides therefrom and temperatures of these exhaust gases are generally low, it is desirable to develop a method in which nitrogen oxides can be removed effectively under such reaction conditions as a lower temperature and a higher space velocity. Further, if ammonia added as the reducing agent is discharged into open air in the unreacted state, there is a fear that it causes another environmental pollution. Accordingly, it is also desirable to develop a method in which no unreacted ammonia is discharged.